In the manufacture of high-bulk tissue products, such as facial tissue, bath tissue, paper towels, and the like, it is common to use one or more throughdryers for partially drying the web or to bring the tissue web to a final dryness or near-final dryness. Generally speaking, throughdryers typically include a rotating cylinder having an upper deck that supports a drying fabric which, in turn, supports the web being dried. In particular, heated air is passed through the web in order to dry the web. For example, in one embodiment, heated air is provided by a hood above the drying cylinder. Alternatively, heated air is provided to a center area of the drying cylinder and passed through to the hood.
When incorporated into a papermaking system, throughdryers offer many and various benefits and advantages. For example, throughdryers are capable of drying tissue webs without compressing the web. Thus, moisture is removed from the webs without the webs losing a substantial amount of bulk or caliber. In fact, throughdryers, in some applications, may even serve to increase the bulk of the web. Throughdryers are also known to contribute to various other important properties and characteristics of the webs.
The use of throughdryers, however, can be expensive. For instance, in addition to the capital costs associated with the equipment, throughdryers have relatively high energy requirements. Therefore, a need currently exists for a system and process for reducing the energy costs associated with throughdryers, while still retaining all the benefits and advantages to using throughdryers.
In this regard, in the past, those skilled in the art have attempted to prevent the heated air used to dry tissue webs from leaking out of the throughdryer and also have attempted to prevent cooler ambient air from leaking into the throughdryer during use. In order to prevent leaks, throughdryers have been equipped with internal seals and baffles. The internal seals and baffles have been placed, for instance, adjacent to an open end of the drying cylinder, where the drying cylinder is not covered by the hood.
Although internal baffles have shown to be somewhat effective at sealing the throughdryer, cool air still becomes entrained in the drying cylinder, which typically has a substantial thickness. This carryover of air corresponding to the thickness of the drying cylinder is commonly referred to as “shell dump”. As the drying cylinder rotates, cool ambient air is drawn into the throughdryer reducing the efficiency of the process. Thus, a need currently exists for a system and process for minimizing shell dump.